This invention relates to timing advance measuring devices, and more particularly relates to stroboscopes used to measure the timing advance of an internal combustion engine.
Many types of internal combustion engines include chambers in which members are movable. For example, gasoline engines typically include a number of cylinders in which pistons move up and down. A gas-air mixture is introduced into the cylinders, and the mixture is compressed by the upstroke of the piston in the cylinder. Just prior to the time the piston reaches the top of its stroke in the cylinder (known as the top dead center position), a spark plug ignites the gas-air mixture in order to drive the piston down.
In order to properly tune such an engine, it is important to know very precisely the number of rotational degrees prior to the top dead center position at which the spark plug fires (i.e., the spark advance of the engine). One conventional way of measuring the spark advance is by use of a stroboscope which flashes within a controllable delay period after the spark plug fires. In normal use, the flash is delayed until the moment at which the piston is in its top dead center position. This position is normally indicated by the alignment of two marks, one located on the fly wheel or pulley of the engine, and the other on the crankcase of the engine. If the stroboscope flashes at the time the two marks are aligned, then the delay between the time the spark plug fires and the time the stroboscope flashes represents the measured spark advance, which is normally expressed in degrees. One such stroboscope device is described in U.S. Pat. No. 3,368,143 (Roberts et al. - Feb. 6, 1968).
In prior art devices, the spark advance is displayed in terms of mechanical degrees that are measured by an adjustable delay time period. As a result, the spark advance degree indication is only accurate as long as the engine is held at a constant speed. If the speed is varied, the indication given by the stroboscope delay time period inaccurately describes the spark advance in terms of mechanical engine position or degrees.
In order to overcome the deficiency of the prior art stroboscope devices, the applicant has invented a technique for flashing the stroboscope which is independent of the speed of the engine. By using this technique, the speed of the engine can vary over a considerable range without affecting the accuracy of the spark advance degrees indicated by the stroboscope.
In order to achieve this result, the applicant utilizes a lamp which illuminates the indicia on the engine in response to a strobe signal. A trigger signal is generated in response to the operation of a combustion means, such as a spark plug, which creates the combustion of the fuel in the engine. Means are provided for storing a variable timing voltage, and a reference voltage is generated. A control device responsive to each trigger signal alters the timing voltage in a first direction at a first predetermined rate so that the timing voltage attains a predetermined relationship with respect to the reference voltage. The control device also generates a strobe signal each time the timing voltage attains the predetermined relationship with respect to the reference voltage in order to flash the lamp.
When the lamp flashes, the control device begins to alter the timing voltage in a second direction opposite the first direction at a second predetermined rate. This second alteration of timing voltage continues until the next trigger signal is generated, such as by the firing of the spark plug. Means are provided for altering the rate at which the timing voltage changes in the first direction. As a result, the flash on the lamp can be adjusted in time until the indicia marks on the engine appear to be lined up. A scale is connected to the adjustment means for displaying the degree of movement of the engine in order to indicate the retard or advance of the timing.
By using the foregoing techniques, the timing of an engine can be measured with a degree of accuracy and reliability heretofore unattainable.